<style>
    #grid{
        width: 1000px;
        border: 2px solid #000;
        margin: 100px auto;        
    }
    #grid div{
        display: flex;
    }
    #grid span{
        padding: 10px;
        margin: 10px;
        background-color: #f0f0f0;
        text-align: center;
        flex: 1;
    }
</style>

<body>

    <div id="grid">
        <div>
            <span>1/2</span>
            <span>1/2</span>
        </div>

        <div>
            <span>1/3</span>
            <span>1/3</span>
        </div>

        <div>
            <span>1/4</span>
            <span>1/4</span>
            <span>1/4</span>
            <span>1/4</span>
        </div>
    </div>

</body>